Nanotechnology/Seeing Nano

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The eyes in nanotech

Without being able to 'see' the nanoscale objects, nanotechnology would be very difficult. In this part, the different microscope techniques are reviewed along with various spectroscopic and diffraction methods that can tell us more about the nanoscale structure of matter.

the electromagnetic spectrum

Visible light is only a part of the electromagnetic spectrum and useful information about different physical interactions in nanostructures can be acquired from the different parts of the electromagnetic spectrum.

Seeing 'nano' can be done in different ways, but not with the naked eye which normally cannot see things much smaller than 100µm (though a single atom can be seen if it lights up in a dark room). Instead of our eyes, we use various instruments to 'see' for us, and they 'see' different things depending on how they are made:

Microscopy

Microscopy uses microscopes to create an image of the specimen. The image is rarely an image as you see it with your eyes, but rather how some physical probe interacts differently with the specimen as function of position on it. The physical probe can be an AFM cantilever, a beam of light or electrons, or something completely different.

Overview of Microscopes

Overview of the different types of microscopes

Optical: The beam from a light source is focused onto a sample and either the transmitted or scattered light is collected by an objective lens and the image is magnified onto a camera or to the observer's eye. The resolution can be down to about 200 nm, and the microscopes can be fairly cheap, small and easy to use.

Transmission Electron Microscope (TEM): Electrons from a very bright electron source are directed to a very thin sample that is transparent to the high energy electrons (100-300 keV) and the electron beam is then magnified by electromagnetic lenses and sent onto a fluorescent screen or a camera to observe the image. The resolution can be less than 0.1 nm on expensive high-end instruments where even individual atoms can be imaged. The samples must be very thin (typically less than 200 nm) and the whole system must be under high vacuum.

Scanning Electron Microscope (SEM): A focused electron beam is scanned over a sample and the scattered electrons are detected. The detector current is used to give an image depending on the electron beam position on the sample. The resolution can be down to about 5 nm and the sample can be much larger than in the TEM because the electrons do not have to pass through the sample.

Scanning Probe Microscopes (SPM) move a very sharp probe across a sample in a raster pattern while recording how the probe interacts with the sample. The typical SPMs are the AFM, STM and SNOM:

Atomic Force Microscope (AFM): An almost atomically sharp tip is protruding from a cantilever and is scanned over the sample. When the cantilever deflects, a laser beam reflected off the backside of the cantilever will change directions and this will be measured by a photodetector. The laser position can be used to control the force between the tip and the sample, and the AFM is often used to measure both topography and forces on the nanoscale. The resolution is normally down to about 1 nm, but even subatomic resolution is possible. The AFM can work with both dry and wet, conducting and isolating samples.

Scanning Tunneling Microscope (STM): An atomically sharp tip is moved within atomic distance of a sample that has a voltage applied to it. When the tip-sample distance becomes so small that the electron clouds of the tip and sample touch, electrons can much more easily tunnel between the two and this gives rise to a tip-sample current (often a few pA at a 1V bias voltage). This current can be used to maintain a fixed tip-sample distance when the tip is scanned over the sample, and this can give images of conducting surfaces with atomic resolution.

Scanning Near-field Optical Microscope (SNOM): As electrons can tunnel between electrical conductors in the STM, photons can tunnel between optical guiding structures. The SNOM uses a narrow light guide to measure how the optical electromagnetic field changes as the guide is moved across the sample. For instance, light can be sent from below the sample and then scattered into the scanning light guide above it. The resolution can be much smaller than the wavelength of light.

Point-Projection Microscopes: The Field Emission Microscope (FEM), Field Ion Microscope (FIM) and the atom probe are examples of point-projection microscopes where ions are excited from a needle-shaped specimen and hit a detector. The Atom-Probe Tomograph (APT) is the most modern incarnation and allows a three-dimensional atom-by-atom (with chemical elements identified) reconstruction with sub-nanometer resolution.

Spectroscopy

Spectroscopy uses spectrometers to tell how radiation interacts with the specimen as function of the energy/wavelength of the radiation

Diffraction

Diffraction uses radiation to observe how it is scattered in different directions from the specimen. This can be used to tell about the order of the atoms in the sample.

Surface analysis

Many of these methods are used for 'macroscopic' surface analysis where the outmost nanometers of a material is being studied over larger areas. The methods can be combined with microscopes to give spectrometrical information from a well defined location on the sample - for instance when doing diffraction measurements in a TEM or level spectroscopy in an STM on a single atom.

References

See also notes on editing this book about how to add references Nanotechnology/About#How_to_contribute.

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